Recently, the number of patients who begin to undergo dialysis therapy because of diabetic nephropathy is increasing year by year. According to statistical data in 1994, the number of patients with diabetic nephropathy who have newly started dialysis therapy in one year was 9,351, which is very close to the corresponding number 10,995 in patients with chronic glomerulonephritis (Akira Sekikawa et al., The 41st Annual Meeting of Japan Diabetes Society) Unfortunately, however, dialysis therapy for diabetic patients causes many problems such as cardiac failure, raising or lowering of blood pressure, infections or shunt troubles, and the average duration of life after the introduction of dialysis is as short as about 3 years (see, for example, Statistical Survey Committee of the Japanese Society for Dialysis Therapy, Journal of the Japanese Society for Dialysis Therapy, 25, 1095–1103, 1992). Thus, it is believed that early treatment by early diagnosis is important for diabetic nephropathy.
In diagnosis and disease state management for renal diseases such as diabetic nephropathy and glomerulonephritis that cause glomerular lesions, clearance tests using inulin, creatinine, etc. are considered bo be useful for the evaluation of glomerular filtration ability. However, clearance tests require timed urine. Also, in clearance tests where an exogenous substance such as inulin is administered, intravenous injection of the exogenous substance is required. Therefore, due to the time and labor required for such tests, application of a clearance test has been limited to patients with apparent renal diseases. Accordingly, it is rather rare to use a clearance test as a routine diagnosis method for those patients who are suspected of renal diseases or who may possibly have renal diseases. In many cases, diseases of such patients have been diagnosed by detecting persistent proteinuria or determining serum creatinine concentrations. However, it is also known that irreversible glomerular lesions have already progressed when persistent proteinuria appears or when serum creatinine concentrations increase (see, for example, “Nephrology”, Kiyoshi Kurokawa (ed.), 249–251, 1995). Therefore, the detection of persistent proteinuria or the determination of serum creatinine concentration cannot be effective means to detect early-stage lesions.
Recently, it has become possible to determine various proteins excreted into urine. For example, it has become clear that albumin excretion in urine increases in diabetic nephropathy, i.e. microalbuminuria precedes persistent proteinuria. At present, it is a general practice to diagnose renal diseases with the appearance of microalbuminuria or the like.
Generally, diabetic nephropathy and other nephropathy are classified into the 1st stage (pre-nephropathy stage), the 2nd stage (early nephropathy stage), the 3rd stage (apparent nephropathy stage), the 4th stage (renal failure stage) and the 5th stage (dialysis therapy stage) (Yukio Shigeta et al., “1991 Diabetes Survey Report”, Ministry of Health and Welfare, 317–320, 1992). The stage at which microalbuminuria is detected falls under the above-described 2nd stage (early nephropathy stage). Pathologically, mild to medium grade diffuse lesions are already present at this stage; the presence of nodular lesions is also known at this stage. For example, diabetic nephropathy is characterized by thickening of the glomerular basement membrane and expansion of the mesangial region. However, it is known that these changes are already present even at a stage when no microalbuminuria appeares clinically (Shigeki Inomata et al., Journal of the Japan Diabetes Society, 30, 429–435, 1987; Bangstad, H., J. et al., Diabetologia, 36, 523–529, 1993). Therefore, the 2nd stage (early nephropathy stage) does not necessarily mean the beginning of nephropathy, and is understood as the stage at which nephropathy becomes diagnosable by current clinical tests. On the other hand, it is believed that those abnormalities occurring at the 1st stage (pre-nephropathy stage) prior to the appearance of microalbuminuria can only be detected by renal biopsy. However, since renal biopsy is invasive, it involves pain and danger. Besides, it requires enormous labor and time from the beginning of biopsy to the obtainment of results. Therefore, the development of a simple, non-invasive test method is desired which can detect those abnormalities occurring prior to the appearance of microalbuminuria.
Under such circumstances, it was reported that urinary type-IV collagen excretion increases even at the pre-nephropathy stage, and the possibility that urinary type-IV collagen could be an earlier indicator of diabetic nephropathy has been suggested (Hayashi, Y. et al., Diabetic Medicine, 9, 366–370, 1992; Yagame, M. et al., J. Clin. Lab. Anal., 11, 110–116, 1997). It is believed that such increase of urinary type-IV collagen excretion reflects histological changes such as thickening of the glomerular basement membrane and mesangial expansion, and that the increase is the result of enhanced production of type-IV collagen in the glomerular basement membrane, glomerular epithelial cells or tubular epithelial cells (Motohide Isono et al., Journal of the Japan Diabetes Society, 39, 599–604, 1996). However, it is considered that even before the above-mentioned histological changes take place in the glomerulus, various changes in metabolism, morphology, etc. are occurring at the cell level, corresponding to hyperglycemia characteristic in diabetes. For example, up-regulation of protein kinase C activity has been reported in the glomeruli of diabetic rats, and glomerular or mesangial cells cultured under high glucose concentrations. The relation between this rise and histological changes in diabetic complications such as diabetic nephropathy is attracting attention (Craven, P., A. and DeRubertis, F. R., J. Clin. Invest., 83, 1667–1675, 1989; Williams, B. and Schrier, R. W., Diabetes, 41, 1464–1472, 1992). Therefore, if it is possible to detect these qualitative changes at the cellular level earlier, more effective treatment could be given before histological changes have occurred. However, no reports have been made to date which clinically examine such utility.
On the other hand, human lipocalin-type prostaglandin D synthase (hereinafter, referred to as “L-PGDS”) is an enzyme which catalyzes the isomerization of PGH2 (a common precursor of various prostaglandins) to PGD2 that exhibits various physiological actions such as sleep induction (Urade, Y., Fujimoto, N. and Hayaishi, O., J. Biol. Chem., 260, 12410–12415, 1985; Urade, Y., Watanabe, K. and Hayaishi, O., J. Lipid Mediator Cell Signaling, 12, 257–273, 1995). Recently, it was revealed that this L-PGDS is identical with β-trace which was known to be present in human cerebrospinal fluid (CSF) abundantly (Hoffmann, A., Conradt, H. S., Gross, G., Nimitz, M., Lottspeich, F., and Wurster, U., J. Neurochem., 61, 451–456, 1993; Zahn, M. Mader, M., Schmidt, B., Bollensen, E. and Felgenhauer, K., Neurosci. Let., 154, 93–95, 1993; Watanabe, K., Urade, Y., Mader, M., Murphy, C. and Hayaishi, O., Biochem. Biophys. Res. Commun., 203, 1110–1116, 1994).
In 1969 when L-PGDS was still called β-trace, Ericsson et al. published a paper suggesting correlation between renal diseases and β-trace (L-PGDS) (Ericsson, J., Link, H. and Zettervall, O., Neurology, 19, 606–610, 1969). Since assay methods at that time were technically immature, L-PGDS was undetectable in serum and urine from healthy subjects, but it was detected in serum and urine from patients with renal diseases such as chronic glomerulonephritis in which abnormalities are recognized in serum creatinine concentration and creatinine clearance. It was suggested that L-PGDS concentrations increase in those patients. Felgenhauer et al. also reported that serum L-PGDS, which was not detected in healthy subjects, was detected though in only one patient with renal failure (Felgenhauer, K., Schadlich, H. J. and Nekic, M., Klin. Wochenschr., 65, 764–768, 1987). Against these findings, Whitsed et al. developed a more sensitive assay system, compared urinary L-PGDS concentrations (amounts excreted/24 hr) from healthy subjects and those from patients with renal diseases presenting proteinuria, and reported that patients with renal diseases not necessarily exhibited higher L-PGDS concentrations (Whitsed, H., and Penny, R., Clin. Chim. Acta, 50, 111–118, 1974). It is believed that the reason why researchers have such opposite opinions on the correlation between renal diseases and L-PGDS is because the assay system used in these reports are semi-quantitative methods based on classical immunological methods using polyclonal antibodies. Recently, using more quantitative assay system, Hoffmann et al. have confirmed that serum L-PGDS concentrations in patients with end-stage renal failure (at dialysis therapy stage) are remarkably increased as compared to the concentrations in healthy subjects (Hoffmann, A., Nimtz, M. and Conradt, H. S., Glycobiology, 7, 499–506, 1997). However, although they use monoclonal antibodies with clarified specificity, their assay method is complicated. Briefly, they purify L-PGDS from serum samples, and then compare the strength of the bands on Western blot. Thus, it is considered to be still difficult to accurately compare minor differences in concentration by this method.
As described above, any of the examinations concerning the correlation between renal diseases and L-PGDS made to date has merely detected a remarkable rise of L-PGDS concentrations in those patients who have been confirmed to have an evidently advanced renal disease by existing diagnostic methods, e.g. showing abnormality in creatinine kinetics, presenting proteinurea or being at dialysis therapy stage. On the contrary, no examinations have been made to date concerning L-PGDS concentrations prior to the progress of renal diseases.